1. Field of the Invention
The present invention relates to the preparation of cellulose fiber pulp from wood and other cellulosic materials. More particularly, the present invention relates to a method and apparatus for continuously monitoring the quantity of dissolved solids remaining in the interstices of a brown pulp stock mat carried by a wash filter surface.
2. Description of the Prior Art
Raw wood, bagasse and other cellulosic fiber sources are delignified by cooking processes in the presence of chemicals which form water soluble compounds and complexes with the natural lignin binder of the raw fiber matrix. Although the chemicals used in the digestive cooking process are relatively inexpensive, those quantities consumed in the 1500 tons of dry pulp per day production of an average pulp mill necessitates an economical recovery and recycle of such chemical values. Moreover, the lignin compounds which must be removed from the cellulose fiber matrix contain sufficient heat value and volatility to contribute favorably to the overall mill heat balance.
The objectives of chemical and heat value recovery from wood cooking liquors are gained simultaneously in a pulp mill recovery furnace. Chemically hydrolyzed lignin, called black liquor, is water flushed from the brown pulp stock on a filter surface which permits the liquor and water to drain from the brown stock while the fibers are supported and retained on the filter.
As washed from the brown stock, black liquor contains approximately 10% to 20% solids in solution and suspension with water. To recover the heat and chemical values present in black liquor, the solids concentration of the solution must be increased to approximately 60%: sufficient to fuel a sustained combustion. This is normally accomplished by evaporation. The 60% solids heavy black liquor is burned in the recovery furnace to release both inorganic chemical values combined therewith and heat for steam generation. A portion of such liquor generated steam is used in a continuous evaporation flow stream for black liquor concentration with the remainder used in support of other mill processes such as paper drying.
This interrelated chemical recovery process is economically dependent on the balance between heat value and water in the black liquor flow stream. Excess water in the liquor stream adds to the heat demand for liquor evaporation thereby reducing the quantity of heat available from lignin fuel to support other mill processes. Such other mill processes must consequently be supported by purchased, supplemental fuels thereby adding dramatically to the overall mill energy costs.
The usual source of such excess liquor water is at the pulp washers, the first objective of most pulp mills being a clean pulp. Excess lignin remaining in the brown stock beyond the washers adds to the bleaching chemical costs or finally, in unacceptable paper quality.
From the foregoing, it should be appreciated that pulp washing efficiency is pivotal to the favorable economics of a pulp mill.
In terms of operating costs, the brown stock washing system presents a trade-off between the costs associated with dissolved solids carryover and the amount of wash water needed to achieve that level of solids carryover. The costs associated with solids carryover are, (1) Chemical makeup needed for cooking liquor, (2) Chemical demand in the chlorination bleaching stage caused by dissolved solids in the liquor, and (3) Heat value of the lost solids which would have been burned in the recovery boiler. Wash water usage costs directly determine steam costs in the evaporators. Reducing the amount of wash water used increases the solids carryover and vice versa and there is an optimum operating condition that minimizes the above costs. Continuously monitoring the levels of dissolved solids carryover from the washing system and adjusting the wash water usage is the most effective way to operate economically.
The present state of the art includes several techniques for monitoring the quantity of dissolved solids remaining in a particular brown stock product. One such technique is an estimation of pulp mat dissolved solids levels by differential inference. Hydrometric devices measure the specific gravity of filtrate to the evaporators. From specific gravity, solids content of the washer filtrate is inferred. By assuming an average quantity of dissolved solids originally present in the brown stock, the inferred value of solids removed with the filtrate is deducted from the assumed original value to conclude the quantity of dissolved solids remaining in the brown stock.
The above state-of-the-art technique does not accurately measure the level of solids carryover from the washer for two primary reasons. First, there is a substantial time difference between the moment filtrate is drawn from a particular flow increment of brown stock and the moment the respective filtrate flow increment is hydrometrically measured for solids percentage. Secondly, the hydrometric data on evaporator entry solids is highly distorted by large quantities of dilution water whereby a very small decrease in the percentage of solids to the evaporators represents a very large increase in solids carryover in the brown stock.
Another technique for determination of the dissolved solids concentration remaining in washed brown stock is by measuring electrical conductivity in the final washer filtrate. Since conductivity of the filtrate indicates the concentration of dissolved solids therein, by differential inferrence the dissolved solids carryover is deduced. J. R. Lavigne, An Introduction to Paper Industry Instrumentation, pg. 344, Miller Freeman Publication, San Francisco, Calif., 1972. See also R. V. Gossage and J. M. McSweeney, Correlation of Solids and Soda Content with Conductivity in Brown Stock Washer Systems, TAPPI Vol. 60, No. 4, Apr., 1977 pg. 110.
Electrical conductivity of mat liquor has also been used for shower water control. In such case a batch sample of pulp mat is taken manually from the flow stream for manual liquor expression therefrom. The liquor so expressed is tested for conductivity and the shower water flow regulated accordingly.
Although it has been known that electrical conductivity of brown stock liquor is related to liquor dissolved solids, the test has heretofore only been applied to relative low consistency, aqueous systems such as the washer drum mixing vat, the washer filtrate or batch samples of mat liquor after manual separation from respective pulp by expression.
An objective of the present invention, therefore, is to provide a continuous and immediate measure of solids carryover in brown stock directly from the pulp mat on the drum filter screen.
Another object of the present invention is to measure the solids carryover in brown stock respective to a particular washer.
A further object of the present invention is to provide a direct and immediate method of evaluating performance quality and efficiency of a particular brown stock washer.